Edexcel IGCSE Physics · Spec 1.1
Units & Physical Quantities
The SI units and physical quantities used throughout IGCSE Physics.
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Units & Physical Quantities
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Explained
Units, and the marks that depend on them
A measurement is a number and a unit together. Written on its own, 12 says nothing about whether it is a mass, a distance or a time. This looks like the least interesting topic in the specification, and it quietly costs more marks than almost any other.
The seven you must know
- Mass, in kilograms, kg.
- Distance, in metres, m.
- Time, in seconds, s.
- Speed, in metres per second, m/s.
- Acceleration, in metres per second squared, m/s squared.
- Force, in newtons, N.
- Gravitational field strength, in newtons per kilogram, N/kg.
Notice what the compound units are telling you. Speed is a distance divided by a time, and its unit says so. Acceleration is a change of speed divided by a time, so its unit is metres per second, per second. If you forget an equation, the unit often reminds you of its shape.
Capital letters matter
Units named after a person take a capital letter as a symbol but stay lower case when written out. So the unit is the newton, written N, and the unit is the kelvin, written K. Metres, seconds and kilograms are m, s and kg, all lower case.
The awkward one is kg, where the k is a prefix and the g is the gram. It is the only base unit with a prefix built into it.
Prefixes and converting
Kilo means a thousand, so 1 km equals 1000 m. Centi means a hundredth, so 100 cm equals 1 m. Milli means a thousandth, so 1000 mm equals 1 m and 1000 g equals 1 kg.
Write the relationship down before converting rather than trying to remember which way to go. Then sanity check: a distance in metres should be a bigger number than the same distance in kilometres, because metres are smaller.
What examiners say about this topic
The summary advice in a principal examiner report for Edexcel International GCSE Physics tells candidates to know the SI units for physical quantities and to be able to convert from non SI units to SI units when required.
The reports then show the cost of not doing it. In one paper, some candidates failed to convert centimetres to metres, which meant the calculation of energy was incorrect because it could not result in joules. In another, candidates were told that derived units such as newtons require inputs in SI units, and that converting grams to kilograms at the start is a good strategy for gaining marks even if the solution cannot be completed. A further report notes learners losing marks for not mentioning units, or for using incorrect units for pressure.
Significant figures appear just as often. One report records that many candidates did not read the question carefully and failed to give their answer to the required two significant figures, after completing the physics correctly.
Three habits worth building
Convert everything to SI units before you start calculating, not afterwards. Write the unit on your final answer every time, even when the question does not remind you. And reread the question for an instruction about significant figures or standard form before you move on, because that instruction is usually printed once and easy to miss.
None of this requires understanding anything new. It is the cheapest set of marks in the paper, and it is available in every calculation you will ever be asked to do.
Spec 1.1
What you need to know
- Give the units of common physical quantities
- Use unit prefixes like kilo, centi and milli
- Convert between units
Active recall
Quick check
Answer each question before opening the answer.
Give the SI units for distance, time and mass.
Distance — metre (m); time — second (s); mass — kilogram (kg).
What are the units of force and energy?
Force — newton (N); energy — joule (J).
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